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1.9.8
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BDE 4.14.0 Production release
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Provide a thread-safe observer that emits log records to a file.
This component provides a concrete implementation of the ball::Observer protocol, ball::FileObserver2, for publishing log records to a user-specified file. The following inheritance hierarchy diagram shows the classes involved and their methods:
A ball::FileObserver2 object processes the log records received through its publish method by writing them to a user-specified file. Note that the enableFileLogging method must be called to enable logging since logging to a file is initially disabled following construction. The format of published log records is user-configurable (see {Log Record Formatting} below). In addition, a file observer may be configured to perform automatic log file rotation (see {Log File Rotation} below).
ball::FileObserver2 offers several manipulators that may be used to configure various aspects of a file observer object. These are summarized in the following table along with the accessors that can be used to query the current state of the configuration. Further details are provided in the following sections and the function-level documentation.
In general, a ball::FileObserver2 object can be dynamically configured throughout its lifetime (in particular, before or after being registered with a logger manager). However, note that for ball::FileObserver2, configuration changes that affect how records are logged (e.g., enablePublishInLocalTime and disablePublishInLocalTime) impact only those records that are published subsequent to making the configuration change.
By default, the output format of published log records is:
where DATE and TIME are of the form DDMonYYYY and HH:MM:SS.mmm, respectively (Mon being the 3-letter abbreviation for the month). For example, assuming that no user-defined fields are present, a log record will have the following appearance when the default format is in effect:
The default format can be overridden by supplying a suitable formatting functor to setLogFileFunctor. For example, an instance of ball::RecordStringFormatter conveniently provides such a functor:
The above statement will cause subsequent records to be logged in a format that is almost identical to the default format except that the timestamp attribute will be written in ISO 8601 format. See ball_recordstringformatter for information on how format specifications are defined and interpreted.
Note that the observer emits newline characters at the beginning and at the end of a log record by default, so the user needs to add them explicitly to the format string to preserve this behavior.
Also note that in the sample message above the timestamp has millisecond precision (18MAY2005_18:58:12.076). If microsecond precision is desired instead, consider using either the D or O format specification supported by ball_recordstringformatter .
By default, the timestamp attributes of published records are written in UTC time. To write timestamps in local time instead, call the enablePublishInLocalTime method. Note that the local time offset is calculated using the UTC timestamp of each record. To revert to UTC time, call the disablePublishInLocalTime method. Whether UTC time or local time is in effect can be queried via isPublishInLocalTimeEnabled. However, note that if the user installs a log record formatting functor via setLogFileFunctor, then the supplied functor determines how record timestamps are rendered to the log.
The calculation of the local time offset adds some overhead to the publication of each log record. If this overhead is an issue, it can be mitigated by installing a high-performance local-time offset callback for bdlt::CurrentTime in main. See bsls_systemtime for the details of installing such a callback and see baltzo_localtimeoffsetutil for an example facility. Note that such callbacks can improve performance for all users of bdlt::CurrentTime, not just the ball logger.
The enableFileLogging method supports the use of %-escape sequences to specify log filenames. The recognized sequences are as follows:
The date and time elements of the derived filename are based on the time when the log file is created. Furthermore, these elements are based on either UTC time or local time depending on the value returned by isPublishInLocalTimeEnabled. (See {Log Record Timestamps} for the similarity with the treatment of record timestamps.)
For example, a log filename pattern of "task.log.%Y%M%D_%h%m%s" will yield the filename task.log.20110501_123000 if the file is created on 01-May-2011 at 12:30:00 local time (assuming enablePublishInLocalTime was called).
A ball::FileObserver2 may be configured to perform automatic rotation of log files based on simple file rotation conditions (or rules).
Rotation rules may be established based on the size of the log file (i.e., a "rotation-on-size" rule), and a periodic time interval (i.e., a "rotation-on-time-interval" rule). These rules are independently enabled by the rotateOnSize and rotateOnTimeInterval methods, respectively. If both rules are in effect, log file rotation is performed whenever either rule applies.
When a log file is rotated, a new filename is generated using the pattern supplied to enableFileLogging. If the file having the new name does not exist, the current log file is closed, and the logging continues to the new file.
If the file having the new name already exits, then the behavior of the file rotation is further controlled by the flag set with suppressUniqueFileNameOnRotation:
suppressUniqueFileNameOnRotation(false) (default behavior) The current log filename is renamed by appending a timestamp in the form ".%Y%M%D_%h%m%s" where the timestamp indicates when the file being rotated was last opened (the time of either the last file rotation or the last call to enableFileLogging, whichever is most recent). As with the timestamps of logged records, the timestamps appended to log filenames upon rotation will be in UTC time or local time depending on the value returned by isPublishInLocalTimeEnabled.suppressUniqueFileNameOnRotation(true) The logging continues to the current log file, effectively suppressing log filename rotation. This may happen when the log file pattern does not contain %-escape sequences indicating a time period, or the rotation interval is less than the time period encoded by %-escape sequences. In order to rotate log files in this mode, the log file pattern MUST contain %-escape sequences that specify date and (optionally) time. For example, the log filename pattern "app_%Y%M%D.log" will produce a single log file per calendar day (assuming, the rotation on time is enabled and the rotation happens at least once a day).The two tables below illustrate the names of old and new log files when a file rotation occurs. We assume that the log file is rotated on 2011-May-21 at 12:29:59 local time and that the last rotation occurred at 12:30:00 on the previous day. We further assume that enablePublishInLocalTime was called, so that all date and time elements are rendered in local time.
The first table shows the name change (if any) of the (old) log file being rotated when the flag controlled by suppressUniqueFileNameOnRotation is set to false:
Note that upon rotation a timestamp was appended to the name of the rotated file when the log pattern does not contain %-escape sequences indicating a time period ("a.log"), or the rotation period (in our case, one day) is less than the time period encoded in the pattern (in case of "a.log.%Y%M" the period is one month).
The next table shows the rotated name when the flag controlled by suppressUniqueFileNameOnRotation is set to true, and (possibly new) name of the (new) log file following rotation:
Note that the original filename is reused when the log pattern does not contain %-escape sequences indicating a time period ("a.log"), or the rotation period (in our case, one day) is less than the time period encoded in the pattern (in case of "a.log.%Y%M" the period is one month).
Also note, that in any cases, when the log pattern includes "%T", or encodes a time period that coincides the rotation period (in case of "a.log.%Y%M%D" the period is one day), then a unique name on each rotation is produced with the (local) time at which file rotation occurred embedded in the filename.
All methods of ball::FileObserver2 are thread-safe, and can be called concurrently by multiple threads.
This section illustrates intended use of this component.
First, we create a ball::LoggerManagerConfiguration object, lmConfig, and set the logging "pass-through" level – the level at which log records are published to registered observers – to DEBUG:
Next, create a ball::LoggerManagerScopedGuard object whose constructor takes the configuration object just created. The guard will initialize the logger manager singleton on creation and destroy the singleton upon destruction. This guarantees that any resources used by the logger manager will be properly released when they are not needed:
Next, we create a ball::FileObserver2 object and register it with the ball logging system;
Next, we configure the log file rotation rules:
Note that in this configuration the user may end up with multiple log files for a specific day (because of the rotation-on-size rule).
Then, we enable logging to a file:
Finally, we register the file observer with the logger manager. Upon successful registration, the observer will start to receive log records via the publish method:
1.9.8